1. Field of the Invention
The present invention relates to a spark plug for use in, for example, a direct-injection engine.
2. Description of the Related Art
In spark plugs for use in, for example, vehicle gasoline engines, a structure incorporating a parallel ground electrode and a plurality of sub ground electrodes has been proposed as a structure for preventing conductive components, such as carbon, from depositing on an insulator.
The parallel ground electrode and sub ground electrodes are provided around a center electrode. Each sub ground electrode opposes the lateral peripheral surface of the center electrode. In the spark plugs of this type, spark discharge occurs between the center electrode and the sub ground electrodes. Spark discharge burns out attached conductive components, such as carbon.
The ends of the parallel ground electrode and sub ground electrodes are not positioned on the same plane (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2001-110546).
Further, to increase the life duration of the spark plug, a structure including a plurality of ground electrodes has been proposed. In this case, when one ground electrode has worn out due to spark discharge, another ground electrode is used for spark discharge. Thus, the life duration of the spark plug is increased.
The ends of these ground electrodes oppose the side surface of a center electrode. Accordingly, spark discharge between each ground electrode and the center electrode occurs on a plane perpendicular to the axis of the spark plug. Further, the ends of the ground electrodes are positioned on substantially the same plane (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 4-196080).